Lawn care calendar for Oklahoma: when to fertilize and seed

Heights and dates are from the OSU landscape maintenance schedule; the lawn management fact sheet gives seasonal heights for other grasses, covered in the text.

Which grasses grow where in Oklahoma

OSU Extension lists bermudagrass as statewide, though it can winterkill in the north, buffalograss in the central and western parts, zoysiagrass in the central and eastern parts, St. Augustinegrass along the Red River in the south, and tall fescue statewide. Buffalograss and bermudagrass need full sun and little extra water. Tall fescue tolerates shade, and OSU rates its water need as high.

For mowing, the lawn management fact sheet gives St. Augustinegrass 2.5 inches from May to August and 3 from September to April, tall fescue 3 inches in summer and 2.5 the rest of the year, and buffalograss 1.5 to 3 inches. Warm-season grasses are cut taller in fall to insulate them from cold, and lower in summer to keep the turf tight.

Frost dates from NOAA

Dates are the NOAA 1991-2020 normals for the last spring and first fall 32 F freeze at 50% probability. At Oklahoma City (Will Rogers World Airport) the last spring frost averages April 4 and the first fall frost November 2. At Tulsa International Airport the dates are March 31 and November 4.

Counting the days between them gives about 212 frost-free days at Oklahoma City and 218 at Tulsa (our arithmetic). OSU publishes one schedule for the state, so the two stations named here give a general sense of the state.

Spring

Weed control starts in the cold. OSU puts the first preventer for summer annual weeds between January and mid-March, with a second dose in mid-May or early June. Dormant bermudagrass can be treated for winter weeds with glyphosate plus a broadleaf herbicide in January or early February, when temperatures are above 50 F and the grass is completely tan.

Warm-season lawns can be started from sprigs, plugs or sod from mid-April, and May is the best month for sprigging, plugging or sodding bermudagrass. Seeding of bermudagrass, buffalograss, zoysiagrass and centipedegrass is best from mid-May through the end of June, when the soil is warm and the season is long enough to build winter hardiness. Dethatch warm-season grass before spring green-up, and before crabgrass control.

Summer

Most Oklahoma turf needs about 1 to 2 inches of water a week in summer, enough to wet the soil 6 inches deep, and should be watered when it first shows wilt, with leaves rolling or folding and turning blue-gray. In spring and fall it often needs none, except in a drought. The fact sheet suggests mowing at 2 to 3 inches for warm-season and 3 to 4 for cool-season grass in summer to conserve soil moisture.

Warm-season lawns can be fed through summer if water is available. A top-quality bermudagrass lawn takes 5 pounds of nitrogen over five applications, according to the lawn management guide, and a lower-effort lawn can reduce the summer feedings, especially common bermudagrass and zoysiagrass. Core aeration for warm-season grass happens in summer if compaction needs it.

Fall

The last nitrogen on warm-season grass goes down by September 15, and a preventer against winter annual weeds goes on in late August to mid-September. Fall is the best time to plant cool-season grass, and late September through mid-October is the window for bluegrass, fescue or ryegrass. Areas being converted from bermudagrass to fescue are sprayed with glyphosate from early August.

Cool-season lawns get fed in mid-October and again in the first week of November, with more feedings in March and May. OSU adds that cool-season grasses need most of their annual nitrogen in fall and very little in summer. Overseeding a warm-season lawn with ryegrass for winter color is done in late October, but OSU says warm-season lawns are healthiest if you skip it.

Fertilizer rules and soil tests

I found no Oklahoma rule limiting phosphorus in home lawn fertilizer. OSU tells you to test the soil: its bermudagrass program assumes phosphorus, potassium and pH are satisfactory according to a soil test, and it uses a complete fertilizer in spring.

The OSU schedule works in pounds of nitrogen per 1,000 square feet, and a bag gives a percentage. The lawn fertilizer calculator on this site turns the percentage and your lawn area into pounds of product, whichever mix of grass you grow.